Fix arith.cmm test
Fix arith.cmm test

An error was triggered on the floating point computations because of
rounding errors between the read from the D array, and the C-side fp
computation which used 80-bit precision.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml